1. Field of the Invention
The present invention relates to a radar device that obtains a distance to a target and a relative velocity, in which a pulse width and a range gate interval (width) are changed depending on a travel environment so as to ensure both of a short-distance precision and a long-distance performance without increasing a calculation amount.
2. Description of the Related Art
Currently known radar devices that are mounted on a vehicle and used for an adaptive cruise control (ACC) or a collision mitigation braking device include a radar device of a frequency modulated continuous wave (FMCW) system (hereinafter, referred to as “FMCW radar device”) which may detect the distance to the target and the relative velocity at the same time. The “FMCW system” is one of radar transmitting systems, and calculates a difference in frequency between a transmission wave and a received wave (a reflected wave resulting from reflecting the transmission wave by the target) so as to calculate the distance to an object (target) and the velocity.
In the FMCW radar device, a transmission signal of a continuous wave (CW) is subjected to FM modulation. The frequency of an oscillator is modulated by a triangular wave, and radiated to the outside from a transmit antenna. A signal obtained by reflecting a transmission signal by the target and receiving the transmission signal by a receive antenna undergoes a time delay caused by the distance, and a frequency shift corresponding to the relative velocity. The receive signal that has undergone the frequency shift is mixed with the transmission signal by a mixer to obtain a beat signal. When a beat frequency in a frequency up zone and a beat frequency in a frequency down zone are measured in each modulation cycle, separately, the distance to the target and the relative velocity may be obtained. The above-mentioned technology is generally known in the field of FMCW radar devices.
In the above-mentioned FMCW radar device, when a plurality of targets exist, the beat signal is generated for each of the plurality of targets, making it difficult to know a correspondence relation between each beat signal and each target. Under the circumstances, there has been proposed a radar device in which a frequency modulated signal as the transmission signal is pulsed as a transmission pulse signal, and a received pulse signal is sampled based on a transmission timing of the transmission pulse signal every given period of time to facilitate the correspondence of the plurality of beat signals (see, for example, Japanese Patent Application Laid-Open No. 2009-150707).
In the radar device disclosed in Japanese Patent Application Laid-Open No. 2009-150707, range gates for conducting sampling timing every given period of time from the transmission timing are provided to detect the target in each of the range gates. In this case, the distance range in which the beat signal is caused by the reflected wave from a target may be narrowed down to some extent based on how much time elapses from the transmission timing for each range gate, false detection is reduced. Further, when a period of time required for sampling from the start of measurement data till the end thereof is regarded as a measurement time, the measurement time is changed for each of the range gates so that an optimum distance resolution and an optimum relative velocity resolution may be set depending on the distance to the target, to thereby realize a bifocal radar.
In the configuration of the radar device disclosed in Japanese Patent Application Laid-Open No. 2009-150707, in order to improve a detection precision in short distance, a range gate width needs to be reduced (that is, a sampling timing interval needs to be reduced). This is because the range gate width is reduced to narrow the target distance range of each range gate, as a result of which the distance range in which the beat signal is caused by the reflected wave from a target may be easily narrowed down, and a desired signal may be easily specified from the plurality of beat signals. Further, because the received wave intensity is inversely proportional to the fourth power of the distance, when the range gate width is reduced, a dynamic range required for signal processing particularly in short distance is reduced. As a result, the number of bits required in the signal processing is reduced to downsize a computing unit. Alternatively, because integration or amplification is more easily executed in the signal processing, an S/N ratio in the signal processing is increased, and a target detection precision is improved.
When a target at a long distance is intended to be detected while keeping the short-distance detection precision, the number of range gates needs to be increased. However, an increase in the number of range gates generally leads to an increase in the calculation amount and memories. As the calculation amount increases, the number of required computing units increases, which prevents the radar from being downsized and reduced in price. Unless the number of computing units is increased, a calculation time is increased, and a response of the radar is deteriorated. An increase in the frequency of calculation leads to an increase in power consumption of the radar, and an increase in calorific value. An increase in the memories also prevents the radar from being downsized and reduced in price.
On the other hand, when the target at the long distance is intended to be detected without changing the number of range gates, the range gate width needs to be widened. However, this leads to the deterioration in the short-distance detection precision. The above-mentioned facts make it difficult to reduce the bifocal radar in size and price.